The present invention relates to the general field of attaching blades to a turbine engine rotor disk.
A preferred but non-exclusive field of application of the invention is in particular that of blades made of composite material for the low pressure turbine of a turbojet of the two-spool bypass type.
The low pressure turbine of a turbojet is made up of a plurality of stages, each stage having a nozzle (i.e. a grid of stationary vanes) and a rotor wheel arranged behind the nozzle. Each rotor wheel comprises a rotor disk having a plurality of blades mounted thereon via their roots, with attachment systems serving to hold the blades on the disk.
The blades of turbine rotor wheels are subjected to high levels of external force, such as contact force at a blade tip, aerodynamic force from the gas, and more particularly centrifugal force that is generated by the rotation of the rotor disk. These forces are countered by the retaining force exerted by the rotor disk, with this force passing via systems for attaching blades to the disk.
The forces that need to pass through the system for attaching blades to the rotor disk are very large. The attachment systems must therefore withstand these forces in an environment that presents fluctuating temperatures (temperature varies between ambient temperature and about 700° C.) and in a limited amount of space.
It has also become common practice to make turbine blades out of composite material, and in particular ceramic matrix composite (CMC) material. By way of example, reference may be made to patent application WO 2010/061140, which describes a method of fabricating such blades.
Compared with metal turbine blades, composite material turbine blades nevertheless present drawbacks associated with their system for attaching them to the rotor disk. Known systems for attaching composite material blades to a rotor disk have difficulties in mechanically withstanding the forces that they need to transfer and they deteriorate rapidly in terms of fatigue and oxidation.
This applies in particular to the attachment systems provided by having co-operating shapes between bulb-shaped blade roots and the slots in the rotor disk in which the roots are mounted. With that type of attachment, the retaining forces exerted by the rotor disk on the root of a blade give rise to a compression effect on the layers of fiber texture (i.e. the force is perpendicular to the direction of the layers in the fiber texture) because of the way the fiber texture layers in the composite material are oriented in order to extend in directions that are parallel to the bearing surfaces of the rotor disk against which they press. This causes the blade roots to deteriorate. Furthermore, fabricating a composite material blade with a bulb-shaped root is relatively complex and expensive.